1. Field of the Invention
The invention relates to a steering control system for a vehicle and a steering control method for a vehicle.
2. Description of Related Art
For example, there is known a control method for an electric power steering system as described in Japanese Patent No. 4969368. In this existing control method for an electric power steering system, a steering assist amount is controlled in response to a steering torque detected by a steering torque sensor and a derivative value of the steering torque. In the control method, at the time when the steering torque sensor is abnormal, the steering torque is estimated on the basis of information about a state of a vehicle, acquired via an in-vehicle network of which the sensing interval is longer than the sensing interval of the steering torque sensor, and the estimated steering torque is used instead of the steering torque. Thus, the steering assist amount is controlled on the basis of the estimated steering torque and a derivative value of the estimated steering torque. In the existing control method for an electric power steering system, in controlling the steering assist amount at the time when the steering torque sensor is abnormal, a control gain based on the derivative value of the estimated steering torque is set so as to be larger than a control gain based on the derivative value of the steering torque at the time when the steering torque is normal.
With the above control method for an electric power steering system, at the time when the steering torque sensor is abnormal, it is possible to control the steering assist amount on the basis of the estimated steering torque, estimated on the basis of the information about the state of the vehicle, and the derivative value of the estimated steering torque. Furthermore, in the above control over the steering assist amount, although the sensing interval at which information about the state of the vehicle is acquired via the in-vehicle network is longer than the sensing interval of the steering torque sensor, it is possible to suppress a decrease in the response of steering assistance at the time when the steering torque sensor is abnormal by setting the control gain based on the derivative value of the estimated steering torque such that the control gain based on the derivative value of the estimated steering torque is larger than the control gain based on the derivative value of the steering torque at the time when the steering torque sensor is normal. Thus, a steering feeling close to that during normal times is obtained.
Incidentally, in the above-described existing control method for an electric power steering system, the sensing interval at which information about the state of the vehicle is acquired via the in-vehicle network is longer than the sensing interval of the steering torque sensor, so an update cycle of the estimated steering torque shortens. Therefore, for example, if torque vibrations due to external force are input to the electric power steering system because of a road noise, or the like, aliasing may occur. That is, in the above-described existing control method for an electric power steering system, if a torque detection cycle becomes longer than half the torque vibration cycle, that is, Nyquist's sampling theorem is violated, aliasing occurs. If the estimated steering torque is estimated in a state where aliasing has been occurring, a desired assist amount (assist torque) may not be obtained.
This will be more specifically described with reference to FIG. 12A and FIG. 12B. FIG. 12A shows a case in which, for example, when the waveform of input torque (estimated steering torque) that is input to the existing electric power steering system periodically varies sinusoidally, a torque (estimated steering torque) that is detected at each sampled point at intervals of the torque detection cycle is detected as a constant value larger than an average value indicated by the dashed line due to the influence of aliasing. In this case, the torque (estimated steering torque) detected as a constant value is larger than the average value, so the assist amount may increase and the differential value obtained from the thus detected torque (estimated steering torque) is “0”. Therefore, it is difficult to maintain the response of steering assistance even when a control gain based on the derivative value is increased.
FIG. 12B shows a case in which, for example, when the waveform of input torque (estimated steering torque) that is input to the existing electric power steering system periodically varies sinusoidally, a torque (estimated steering torque) that is detected at each sampled point at intervals of the torque detection cycle is detected so as to fluctuate in accordance with an apparent torque fluctuation cycle due to the influence of aliasing although the average value indicated by the dashed line is constant. In this case, the detected torque (estimated steering torque) apparently fluctuates at a cycle different from the periodical variation (oscillation) of the input torque for the constant average value. Therefore, the assist amount may fluctuate unintentionally, and the derivative value also unintentionally fluctuates, so it is difficult to maintain the response of steering assistance.
In this way, if the torque (estimated steering torque) or the derivative value of the torque, both of which can be influenced by aliasing, is used, it may be difficult to appropriately use, particularly, the derivative value of the torque (estimated steering torque) in steering assistance control. As a result, vibrations (self-excited vibrations) due to the input torque easily occur in a turning operation direction of a steering wheel. Thus, when there occurs an abnormality in the steering torque sensor, a driver may perceive unpleasant self-excited vibrations due to a disturbance, such as a road noise, via the steering wheel gripped by the driver itself, and may experience discomfort.